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Full text of "General illumination course, January 1930 : assignment No. 2 : contents units of light measurement photometry."

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AsBignment 2, Page 21 


General IlluTulnation Course 


Candleijower - light poTrer of light sources 

Even after more powerful and more efficient light sources 


superseded candle light. Illuminating power was still expressed In terms 
of candles. For this reason it tecame general practice to speak of an 
illvirainant as producing as much light, or '^candlepower" , as so many 
candles. Obviously, the ingredients of individual candles, and the 
varying conditions under -which the/ were b\irned, considerably affected 
the amount of light obtained. It was therefore necessary to provide a 
standard of comparison which could be used universally as a "yardstick" 


for measuring the lighting power of any illuminant, 


This was accomplished by specifying that a light source of 

"one candlepower" be the light equivalent of a candle made according to 

definite specifications and burned under certain prescribed conditions. 

In this manner the term "candle power" came into general use as the unit 

of measurement for the intensity of light sources. France, Great 

Britain and the United States have established the "International Candle" 

as the primary standard to be used in all light measurements and cal- 

The candlepower emitted in a given direction gives no 
indication of the total amount of light produced by the illuminant. 
Candlepower read in one direction is ajialogous to the depth of a pool 
of water at a given point - a measurement which is useful for certain 
purposes, but which is of no value in determining the total quantity of 
water in the pool. Just as it is necessary to know the dimensions of 
a pool and the depths at other points before its total contents can be 
established, so it is necessary to know the candlepower of an ill\iminant 
in all directions before its total light output can be determined. 


General Illumination Course 

ABElgnment 2, Page 22 


Fig. 9 - The candlepower in the direction of the photo- 
meter P is not changed by partially surrounding 
the light source with a non-reflecting surface. 


In A, Fig. 9, the photometer P* indicates an intensity of one candle- 
power. In B, the candle is surrounded by a sphere having a moderately 
large opening. Assuming that none of the light rays are reflected 
from the inside walls of the sphere the photometer will still Indicate 
an intensity of one candlepower despite the fact that a large portion of 
the total light from the candle has been absorbed. In C, a sphere 
with a much smaller opening' is illustrated and still more of the light 
is consumed by the sphere, but even in this case the light in the 
direction of the photometer is one candlepower. In fact, the reading 
rill be one candlepower irrespective of the size of the onening and 
regardless of the quantity of light allowed to be emitted, provided the 
direct rays from the candle to the photometer are not obstructed. 

Figure 10 indicates three ways in which candlepower measure- 

mente are ordinarily made. In A, the intensity of light radiating 
in one direction is measured. When a number of readings are taken 

• A photometer is an instrument used to measure the 
candlepower of a light source, or the intensity 
of illumination on a surface. 



General Illuwination Course 

ABBigniLent 2, Page 23 

at iinlform intervale in a horizontal plane, ae indicated in B, and then 
averaged, the result is the mean horizontal candlepo-ftcr of the light 



thle result is obtained in ordinary practice by rotating the illuminant 



intensity of light 


0, by measxiring the candlepower at uniform intervals abound the light 


An average of these readings will give the mean spherical 

candlepower of the illuminant. 

This is a true evaluation of its 

average light intensity or illuminating power. 


spherical candlepower is usually obtained by placing it inside a sphere 

the inner surface of which is painted a flat white. 

A single reading 


window from which the 


This measurenent 

A - Horizontal 

B - 

Mean tiorizontal 


C - Hean spherical 

Fig. 10 - Measurement of candlepower 


Indicates the average candlepower in all directions because of the 
BUltiple reflections of light which occur within the chamber. 



General Illumination Couree 


ABBlgnment 2, Page 24 


The Lumen - quantity of light 

The unit used to denote quantity of light flux Is the "lumen". 
ThlB le the amount of light falling on a surface one square foot In area, 
every point of which is one foot from a source of one candlepower. 

If the opening Indicated in section by OR, Fig. 11, is one 
square foot In area, the light escaping will be one lumen; if this open- 
ing le doubled it will be two lumens. Since the total surface area 
of a sphere with a one foot radius is 12.57 square feet, a uniform 
1 candlepower source of light emits a total of 12.57 lumens. Lumens 
therefore equal mean spherical candlepower times 13.57. Thus a light 
source of 100 mean spherical candlepower enits 1257 l\iraens. 

Fig. 11 - A - Opening OR ha's an area of one square 

foot and allows one lumen to escape. 

B - One lumen falls on surface OPQR. 



The Foot-Candle - Intensity of Illumination 

Light may be termed the cause, and illumination, on the 
other hand, the effect or result. Candlepower is a measure of the 
cause. It therefore applies only to the light source itself and not 
to the effect or result obtained. The unit of measure of illumination 
in the United States Is the "foot-candle**. 

A foot-candle represents the illumination at a point on 
a surface which is one foot distant from and perpendicular to the rays 
of a one candlepower light source. In Fig. 12, if the light source 
8 has an intensity of one candlepower along the line SA, and if A is 



General Illumination Oouree 

Aeeignment 2, Page 25 

one foot difitant fro 

so\irce, the illumination on the Diane 

the point A is one foot-candle. 

The "foot-candle" la the unit of measurement most intimately 

aaeoclated with everyday use of light. 

A working idea of thia unit 

can be obtained by- holding a lighted 
candle one foot distant from a news- 


The result will be approxi- 

mately one foot-candle of illumination 
A foot-candle reading applies only 
to the particular point where the 

Fig. 13 - 

Illumination at A 
is 1 foot-candle. 

measurement is made. 

By averaging 

the foot-candles at a number of points. 

the average illumination on any given surface can be obtained. 

The following table which lists the foot-candle intensities 
experienced in everyday life, will serve as a basis for a better under- 

standing of various levels of illumination. 



Street Lighting (approximately) 
Lighted Store (approximately) 
Industrial Lighting (approximately) 

At North Window 

In Shade (outdoors) 

Direct sunlight (June) 


10 - 15 
10 - 20 

50 - 200 

100 - 1000 


Care should be taken to avoid confusing the degree of lllu- 

Inatlon on, with the brightness of, a surface. 

A gray siirface will 

not appear as bright as a white surface under the same Illumination, 
since white reflects more light to the eye than pray. The brightness 
of an object thus is affected not only by the foot-candles of illu- 

Inatlon on it but also by its reflection factor, i.e., the percentage 
of light it reflects. 

The degree of illumination on a surface depends on the 

candlepower of the illuminant and its distance from that surface. It ie 



General Illumination Couree Aeeignment 2, Page 26 

obvious that if, as illustrated in Fig. 12, instead of an intensity of 


one candlepower being produced along line SA, an intensity of two candle- 
power were produced, the J.lluniination at A would be twice as great, and 
that if there were an intensity of five candlepower, the illumination 
would be five times as great. 

Fig. 13 - The illumination on a surface varies inversely as 

the square of the distance from the source to the 

In considering Fig. 13, if the source of light is one 

candlepower, the Illumination on a spherical surface one foot distant, 
as Illustrated by A, is one foot-candle. If surface A is removed, 
the same amount of light passes to surface B, two feet away, and here 
covers four times the area of A. Since light travels in straight lines 
and none of it is lost, the average intensity on B, two feet away is one- 
fourth as great as that on A, one foot away^ or one fourth of a foot- 
candle. If B is removed and the same amount of light falls on surface C, 
three feet away from the source, it will be spread over an area nine times 
as great as A. The resulting illumination is therefore 1/9 of a foot- 
candle. At a distance of five feet, the illtiminat ion would be only 
one twenty-fifth of a foot -candle. Illumination decreases not in 
proportion to distance, but in proportion to the square of dietaince. In 
general, the illumination produced on a surface by a single source can be 
obtained by dividing the candlepower of the light source by the square 

of its distance from the surface. This relation is commonly known as 
the inverse square lav 



General I Humiliation Course Aeelgnment 3, Page 27 

One lumen utilized bo that all of its light is spread over 
a surface of one square foot will produce an average intensity of one 
foot-candle. Two lumens would produce an average intensity of two- foot- 
candles over this same area. On the other hand, one lumen would only 
furnish an average illumination of 1/3 foot-candle over an area of two 
square feet. This relation greatly simplifies the designing of a 
lighting installation. Ifhen the number of squfure feet to be lighted 
are known and the desired intensity of illumination is decided upon, it 
is a simple matter to determine the number of lumens which must be pro- 
vided on the working plane. If, for example, it is desired to illu- 
minate a surface of 100 square feet to an average intensity of five foot- 
candles, 500 lumens must be distributed xiniformly over the surface. 

Foot-Candles x Area (Sq. Ft.) s Lumens. 

Candlepower per Square Inch - brightn ess 

Any object which emits of reflects light has brightness 
and is visible. In this country brightness in any given direction is 
measured in terms of "candlepower per square inch". Thus the bright- 
ness of a surface in a given direction equals its candlepower in that 
direction divided by its projected area in square inches. 

The following table lists the brightness of typical light 
sources in candlepower per square inch and should facilitate a better 
understanding of this unit of measure for brightness. 

Oas Flcune 
Enclosing Globe 
White Bowl Lamp 
Frosted Lamp 

Candlepower Candlepower 

per 3q. In. 

Bq. In. 

3.5 Mazda C Lamp Filament 6,500 

2.5 - 4.0 Crater of Carbon Arc 100,000 

13 High Intensity Arc 400,000 

50-60 The Sun 1,300,000 





General Illuruination Course 

ASBlgntnent 2, Page 28 

Lighting authorities are in general accord that lighting- glassware sus- 
pended in the ordinary line of vision should not have a surface brieht- 
nesB in excess of 3.5 candles per square inch. 


Photometry is that branch of physics which deals with the 

measurement of light. 

The rcore common methods and instruments employed 

for this purpose are described in the following paragraphs. 

If a hole were cut in the curtain of a theatre and this 
were covered with translucent material, such as waxed paper, a person 
Bitting in the orchestra could readily tell when the stage lights were on 


The small spot would stand out in bright 

contrast to the 6\irrounding surface of the curtain. 

If the illumi- 

nation in the house were gradually increased, the spot would appear less 

and less luminous. 

At a certain point it would have the same bright- 

nesB as its surroundings, appearing neither less nor more luminous than 

the adjacent area. 

On the other hand, if the stage were dark and the 

house were light, the hole in the curtain would appear as a dark spot. 
At the point where the spot seems invisible it indicates that the illu- 

Ination on both sides of the curtain is of the same intensity. 


ability of the human eye to recognize equality of brightness is the 
basis of most photometric measuring devices. 

The SimtJle Bar photometer 

In laboratory practice, the simple bar uhotometer, illustrat- 

ed In Fig. 14, utilizes the principles described above, 
paper screen, indicated by C, corresponds to the curtain. 

A vertical 

In the 

center of this is a grease spot which rriakes a small portion of the screen 

translucent . 

B is a standard source of light providing one candlepower 

In a horizontal direction. 

The horizontal candlepower of lamp A Ib to 



General Illumination Course 

ABslgnment 2, Page 29 

be determined. 

By moving the photometric screen, C, back and forth 

between the two light sources, a point is found where the grease spot 


surrounding paper on the 


In order that the observer may see both sides of the screen 

Fig. 14 - Simple bar photometer 

simultaneously, oblique mirrors axe placed, as shown in Fi?. 15. 


arrangement facilities accuracy and rapidity in testine, especially when 
the two light sources emit light of slightly different colors. 


the screen is in a position where the spot disappears, or the two spots 
as reflected in the mirrors appear equally bright, the photometer is said 
to be balanced. In this position the illumination in foot-candles on 

both sides of the screen is equal. 

The ratio of candlcDower of the two 

light sources then equals the ratio of the squares of the distances from 

each side of the screen. 

Candlepower of A 
Candlepower of B 

(Distance from screen to A) 2 

(Distance from screen to B)^ 

Fig. 15 - Bar photometer showing use of oblique mirrors 

Assuming that the point of balance lies one foot from B 
{Tip-* 15) and the candlepower of B is one, one foot-candle of illumination 

is produced on the side of the screen which faces it. 

Since the 



General Illumination Course Aeeignroent 2, Page 30 

photometer ie balanced the Illumination on the opposite side, obtained 
from lamp A, must also be one foot-candle. Foot-candles decrease as 
shown in Fig. 13, in proportion to the square of the distance for the 
source of illumination. Therefore if lamp a is three feet away and 
gives one foot-candle on the screen, its candlepower is (3) or 9. 


Fig. 18 - Photometer and screens. 

A - Screen at left of balance point. 

B - Screen at balance point. 

C - Screen at right of balance point. 

Fig. 13 illustrates the apuearance of the photometric screen 
at various positions. In A the spot as viewed in the left-hand mirror 
Is darker than its surroundings and as viewed in the right-hand mirror 
is lighter than its surrounding's. This indicates that more light falls 
upon and is transmitted through the left-hand side of the screen than on 
or through the right side. In C the conditions are reversed; the 
illumination on the right side of the screen is greater than that on the 
left. Somewhere between these two positions is a point at which the 
spot ceases to be visible as shown in B. 

The apparatus need In making photometric readinga as 
described in the preceding paragraph ie known as a Eunsen photometer , 
For more accurate photometric measurements the grease spot screen is 
replaced by a somewhat more complex piece of optical apparatus* 




General Illumination Course 

ABelgnment 2, Paige 31 

The Dletributlon Phot ome t er 

Some photometers measure simply the average horizontal 

candlepower of an illurainant. 

To determine the candlepower at angles 

above or below the horizontal, the lighting unit is held in a vertical 


that the photometer bar carrying the comparison screen and the standard 

may be readily tilted up and down in a vertical Diane 


desired angle. 


distribution photometer (Fig. 18) used in making tests of lighting units 
has a movable mirror which redirects the light rays from any paxticular 

angle so that they may be 


e standard lamp (not 

shown in the illustration) on the movable horizontal carriage at the right. 

Candlepower measurements are usually taken with photometers 
of this type at 5° intervals throughout the entire 180° from a point be- 
neath the test unit to a point directly above it. A properly weighted 
average of these readings gives what is termed the mean spherical candle- 
power of the lighting unit. 



732 1 

i * 

725 j 


705 1 

r 2^ 

1 as 


MO 1 





Fig. 17 - Methods of recording candlepcwer measurements 


The tabulation in Fig. 17-a shows the results of a test 
Uazda C lamp equipped with an RLM Standard Dc 

In this table 0° refers to the reading taken directly beneath the unit. 



General Illumination Oouree 

Assignment 2, Page 32 

The candlepower in this direction was found to be 732. 

Likewise 90° 

represents a measurement taken on the horizontal; due to the type of 
reflector chosen the candlepower at this and higher angles was zero. 

In Fip. 


It will be noted that at each angle the 

curve passes through a point which corresponds to the tabulation in 

Fig. 17-A. 

The curve cuts the 0^ line at 732, passes through 35° at 665 

and cuts the 65° line at 380, etc. 
candlepower distribution curve. 

This graph ie commonly known as a 

The area of a distribution curve is not a criterion of 

the total Aight output of a source. 

In Fig. 17-C both curves shown are 

taken from units giving exactly the same total lumens of light. 


distribution curve serves merely to show the candlepower at various angles 

The Sphere Photometer 

As previously mentioned, mean spherical candlepower and lumen 
output may be obtained by a direct reading in a sphere photometer, usually 

* known as an Ulbricht Sphere, illustrated in Fig. 13. 

In this photo- 

eter the lamp to be measured is placed at the center of the large sphere 

the inside wall of which is painted a flat white. 

A small window of 

opal glass is shielded from the direct rays of the lamp by a small opaque 


The Illumination on the window varies in direct proportion to 

an BDherical candlepower of the lamp 

In order to obtain a 

measure of the brightness of this window, a photometer is used in which 
a standard lamp Illuminates a glass disk similar to that used in the 


The illumination on this second disk can be varied at will 

by moving the standard lamp back and forth, and by means of mirrors in 
the photometer the two disks may be viewed simultaneously and balanced 
Thus the total lumens' or mean spherical candlepower of a lamp can be 
determined at one reading. 

are used to 

Larger spheres eimilar in principle to the Ulbricht Sphere 
eaeure the lifirht output of various types of lighting equip 

ent . 

The Westinghouse Icosahedron (the twenty sided enclosure 



General Illumination Couree 

Aeeignment 2, Page 33 

shown in Fig. 18) will measure with two readings the output or efficiency 

of complete lighting units, even the large fixtures used for street light 

Lumens Per Watt - light source efficiency 

It ie necessary to measure not only the candlepower of a 
light source but also its efficiency. In modern electric lamps this 
is expressed in lumens per watt, which is siraply the number of lumens of 
light flux produced divided by the watts of electrical energy consumed. 
For most lamps these values are based on measurements made in the sphere 
photometer . 

The efficiencies of ordinary Mazda lamps in common use vary 
widely. The 10 watt s-11 bulb lamp produces 7.7 lumens per watt as 
compared to 30 lumens per watt for the 10 K.W. G-80 bulb lamp. The 
average efficiency of the lamps used in the United States is somewhat 
over 10 lumens per watt. 


Ability to see depends upon the levels of illumination on 
objects at which we are looking. In our daily use of light, therefore, 
we are more interested in jneasurement s of illumination on the working 
planes than in candleepowers of sources of light. There are a number 
of instruments available (Fig. 19) with which to measure foot-candles 
of illumination on surfaces. 

The Foot-Cand le Meter 

This instrument is a small, conveniently portable photo- 
meter, sufficiently accurate for ordinary illumination measurements. A 
miniature incandescent lamp, operated from a dry-cell battery, is at one 
end of a trough which is covered by a screen having a series of trans- 
lucent spots similar to the grease spot of the simple bar phototLCter. 
When the lamp in the meter is turned on, the spots on the screen are illu 
minated. Their brightness depends upon the distance of each from the 




General Illumination Course Aselgnment 2, Page 34 



(Equivalent to a 
10 foot sphere photo 
Westinghouse Lamp Company 


Distribution Photometer 
Westinghouse Lamp Company 

Ulbricht Sphere photometer 

Fig. 18 - Photometers for measuring light output 

(distribution or efficiency) of incan- 
descent lamps or other lighting units. 




0«neral Illumination Courea 


▲•Blgnment 2, Page 35 


Photo-electric Cell Photometer 

llaCbeth IlluminoMtex 
rig. 19 - Photometers for measuring the Intensitf of illumination. 



General Illumination Course 

Assignment 2, Page 37 


1. In your own worde, what ie your conception of 
the terra candlepower? Foot-candles? Lumen? 

2. Upon what fundamental law is the principle of 
the photoraeter based? 

3. Give, briefly, some of the commercial applications 
of the photometer. 

4. ^Afhat ie the principle of the foot-candle meter? 

5. How does the accuracy of reading and ease of 
manipulation of the foot-candle meter compare 
with that of a laboratory instrument? 


Photometry by John V/.T, Walsh. 

E.P. Dutton & Co., New York City. 

Illuminating Engineering by Cady and Dates. 

John Wiley & Sons, Inc., New Vork City. 

Practical Electric Illumination by Croft. Section 3. 

McGraw-Hill Book Co., New York City. 

Light, Photometry and Illuminating Engineer ine- by Barrows. 

McGraw-Hill Book Co., New York City. 

New Methods of Showing Photometric Eata by S.G. Hibben. 

I.E. 3. Transactions - Vol. 21, pg. 169, 1923. 

A Distribution Photometer of a New Design by C.C. Colby, Jr. 

and CM. Doolittle. 

I.E.S. Transactions - Vol. 18, pg. 273, 1923. 




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